1. Technical Field
This invention relates generally to piston assemblies for internal combustion engines, and more particularly to connecting rods within the assemblies.
2. Related Art
Conventional piston assemblies for use in internal combustion engines generally comprise a crown or head portion which provides a reaction surface for the combustion in a corresponding cylinder of the engine. A pair of laterally spaced pin bosses depend from the piston head and terminate in a pair of coaxial pin bores. A small end of a connecting rod is journaled about a wrist pin, and the wrist pin is journaled within the pin bores.
Modern vehicle engine applications are experiencing increased peak cylinder pressure demands in order to meet ever increasing performance requirements. Unfortunately, the increased peak cylinder pressure demands result in an increased pressure distribution across the pivotal joint connection between the small end of the connecting rod and the wrist pin. The increased pressure between mating bearing surfaces of the connecting rod small end and the wrist pin can result in a reduced useful life expectancy of the mating bearing surfaces. Accordingly, rather than accept the reduced life of the bearing surfaces, the peak cylinder pressures have been reduced.
When the peak cylinder pressures are increased, there are typically two main failure modes. First, the mating bearing surfaces between the wrist pin and the connecting rod small end scuff due to excessive friction. The excessive friction is exacerbated from having substantially straight, cylindrical bearing surfaces, such as that shown in a prior art connecting rod small end PA1 in FIG. 1, being in contact across their entire length, thereby producing maximum pressures at the center region CR of the mating bearing surfaces. The high pressure tends to expel the lubrication axially outwardly in the direction of arrows A, thereby creating a starvation of lubrication in the center region CR. The diminished film of lubrication in the center region CR results in increased friction, which in turn generates heat, and ultimately bearing surface fatigue. Second, bushings, typically inserted within the connecting rod small end bores, fatigue, such as at their ends or outer edges where stress concentrations are highest. Some attempts to combat this phenomenon have been made by creating a convex surface CS extending completely between the opposite ends or edges OE of the connecting rod, such as shown in another prior art connecting rod small end PA2 in FIG. 2. Unfortunately, these efforts, although reducing the pressures at the outer edges OE, still result in a reduced lubrication over the center region CR due to an ever increased pressure gradient in the center region CR.